FL 3 Kommunikationssystem
what is a network
Network: a collection of nodes and link
what is a node
Nodes are either hosts or routers
• Host: end-node that runs software to support applications, e.g., laptop, server, smartphone
• Router: a node that relays and routes messages, e.g., ADSL modem, Wi-Fi access point
what is a link
Link: a physical medium that connects nodes (can include hubs, switches)
what is application
Application (program): uses the network, e.g., Spotify
what is a layer
• A layer is a set of related communication functions that
– provide services to the layer above it
– by using services of the layer below it
what can a service be
Services are either connection-oriented or connectionless; connection-oriented services create logical connections between end nodes before the transmission of higher-layer data
what is a protocal
A protocol is a set of rules that determines how communicating entities in the same layer
interact
describe layer and layer -n entity
• A layer can contain many different protocols but not the opposite
• A layer-n entity is a piece of software or hardware that executes a layer-n protocol
• Layer-n entities at different nodes are called peer processes • Layer-n peer processes communicate by exchanging PDUs (protocol data units)
how do layers communicate
Layers at the same node communicate by exchanging SDUs (service data units) and control information over a SAP (service access point)
• SDUs are encapsulated into PDUs: PDU = header + SDU + trailer
describe function of layer
• A layer can fragment a SDU into smaller pieces at the transmitter and reassemble the SDU at the receiver
• A layer may put several SDUs into a single PDU at the transmitter and unpack the SDUs at the receiver; this is called aggregation
• A layer can serve several entities in the layer above (multiplexing)
• A layer can use several entities in the layer below (splitting)
• Sometimes we use layer-specific names for PDUs
name the PDU of the layer application
message
name the PDU of the layer transport
segment
name the PDU of the layer network
packet
name the PDU of the layer link
frame
describe the OSI Reference Model
- 7,6,5 -> application -> provides services frequently needed by application programs
-4 -> transport -> end-t-end transfer of messages
-3 -> network -> tranfer or packets over several links
-2 -> data link ->transfer of frames over a single link
-1 -> physical -> transfer of bits over a physical channel
describe internet working
• Internetworking is needed to provide connectivity between hosts that are attached to different networks
• If Alice wants to talk to Bob, messages need to traverse five networks that are connected by routers
– We assume here that the switches are transparent, i.e., not noticeable, to the data-link layer in hosts and routers
• In the TCP/IP model, the protocol IP provides internetworking
describe the internet model as in the book
The Internet model is also called the TCP/IP model
5=application
4=transport
3=network
2=data link
1=physical
TCP/IP Protocol Suite
5. Application = DNS, DHCP, HTTP, SMTP
4. Transport= UDP, SCP, IGMP
3. Network= ICMP, IP
1 + 2 Data Link + Physical = Wi-Fi Ethernet
TCP/IP Transport and Network Layer Services
4. Transport
TCP = reliable, connection-oriented , full-duplex, byte-based, stream service
UDP = best effort, connectionless, datagram service
SCTP = reliable, connection-oriented, multihoming, full-duplex, communication over multiple IP message-based services possible, multistream service
3. Network
IP = best effort, connectionless, packet transfer
internet names and internet adresses
Internet names
• Unique and “easy” to remember
• Independent of physical location
• Hostname has two parts: host + domain name
www.(host)chalmers.se(domain)-> DNS translate -> 129.16.17.21 (32 bit for IPv4) net id + host id
Internet Addresses
• An IP address uniquely identifies a network
interface card (NIC)
• A NIC can have several IP addresses
• A node can have several IP addresses, e.g., if attached to several networks
• Routing decisions are based on destination IP addresses
• IP address has two parts: net id and host id
link layer adresses
• LANs (and other networks) assign link-layer (physical, MAC) addresses to the physical attachment to the network
• The link-layer protocol uses link-layer addresses to transfer packets to the appropriate destination on the same network
link layer adressses
• IP addresses need to be resolved (translated) to link-layer addresses before transmission over the network—this is done with ARP (Address Resolution Protocol)
• Example: Ethernet uses 48-bit addresses
– Each Ethernet NIC has a globally unique Medium Access Control (MAC) address =physical address = link-layer address – The first 24 bits identify the NIC manufacturer; the second 24 bits are a serial number
– 24:A2:E1:E6:BF:C4 12 hex numbers = 6 bytes = 48 bits
what are the three main types of link layer adresses
• Three main types
– Unicast (point-to-point): address a single node on the network
– Multicast (point-to-multipoint): address more than one of the nodes on the network
– Broadcast (point-to-multipoint): address all nodes on the network
• Ethernet addressing scheme
– Unicast example: 24:A2:E1:E6:BF:C4 (second hex digit must be even)
– Multicast example: 01:80:C2:00:00:00 (second hex digit must be odd)
– Broadcast: FF:FF:FF:FF:FF:FF
obtain and resolve adresses. Internet names
Internet name: rent domain name from a domain name registrar (e.g., GoDaddy)
obtaining and resolve adresses
IP address: typically dynamically assigned at power up through Dynamic Host Configuration Protocol (DHCP)
obtain and resolve addresses. link layer address
Link-layer address: assigned to NIC at manufacturing
addesss resolution protocal, ARP
To transmit an IP datagram over a link, the IP destination address needs to be translated (resolved) to the corresponding link-layer address